Heretofore, stainless steel-type metals or polycarbonate-type plastics have been used as orthodontic bracket materials. Oxide ceramics such as alumina and zirconia have also been used. The stainless steel-type materials are excellent in strength and machinability but have metallic luster, and they thus had a drawback that a bracket made thereof stood out and spoiled the appearance when attached to a row of teeth. From the viewpoint of overcoming the drawback, a transparent plastic bracket is sometimes used. However, the plastic bracket had a problem that plastics had insufficient durability and underwent discoloration and the rigidity thereof was not enough to achieve satisfactory orthodontic effect.
An alumina ceramic bracket has been developed as one having superior beauty to metals and overcoming the deterioration of durability of plastics. For example, Patent Document 1 discloses an alumina bracket containing at least 85 wt % of Al2O3 and exemplifies a ceramic with a bending strength of 330 MPa obtained by sintering alumina powder at a temperature of from 1,575 to 1,675° C.
Patent Document 2 discloses a highly-pure alumina bracket and describes that the bracket is made of an alumina ceramic material having crystal grain sizes of from 2 to 50 μm, preferably from 10 to 30 μm, and a bending strength of at least 280 MPa. It further discloses the following production methods: a method of sintering the material at a temperature of from 1,750 to 2,050° C. in a hydrogen atmosphere, and a method of subjecting the sintered body to an HIP treatment at a temperature of from 1,750 to 2,050° C. and under a pressure of 100 MPa.
Furthermore, Patent Document 3 discloses a highly-pure alumina bracket containing at least 99.9 wt % of alumina, and describes ceramics having crystal grain sizes of from 1.8 to 3.0 μm and a bending strength of from 530 to 640 MPa in Example. It further describes the following production method: a method of sintering the material at a temperature of from 1,300 to 1,400° C. and under ordinary pressure, and subjecting the resultant to an HIP treatment at a temperature of from 1,400 to 1,550° C. and under a pressure of at least 50 MPa.
Moreover, Patent Document 4 discloses an alumina ceramic bracket of fine crystal grains of no greater than 1.0 μm and a ceramic material having a bending strength of 620 MPa. It describes a method of sintering the material at a temperature of from 1,200 to 1,300° C. and then subjecting the resultant to an HIP treatment at a temperature of from 1,200 to 1,300° C. as a production method for the bracket.
Patent Document 1: U.S. Pat. No. 4,219,617
Patent Document 2: U.S. Pat. No. 4,954,080
Patent Document 3: JP-A-3-168140
Patent Document 4: U.S. Pat. No. 6,648,638